Stellar halos are built largely from the accretion and disruption of satellite galaxies. Tidal debris features from these disruption events remain identifiable for billions of years, providing observable signatures of the merger histories of individual galaxies. However, in situ star formation may also play a part in building up the inner regions of stellar halos, either via stars formed in the proto-disk of the host galaxy, or in gas deposited by disrupted satellites.

Therefore, studying stellar halos in detail provides a window into the formation histories of galaxies. With current instrumentation, the only stellar halos that can be studied in great detail are the Milky Way and Andromeda (aka M31), the two large spiral galaxies of the Local Group.

I am a member of the SPLASH collaboration (Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo). Our team has amassed a large photometric and spectroscopic dataset of red giant branch stars in M31′s halo and dwarf galaxies. Our photometric data are primarily taken with the Mosaic camera on the Mayall 4-m telescope on Kitt Peak, and include narrow band imaging that allows us to select spectroscopic targets with a high probability of being M31 stars. Our spectroscopic data are taken with the DEIMOS multi-object spectrograph on the Keck II 10-m telescope.

Figure 1: The locations of our Keck/DEIMOS spectroscopic fields in M31′s stellar halo, overlaid on the PAndAS starcount map (McConnachie et al. 2009). Our spectroscopic observations target fields on and off halo substructure, and cover a large range in radius.

My recent focus is on leveraging the full M31 halo dataset (shown in Figure 1) to learn about the global properties of M31′s stellar halo, and the ensemble of disrupted dwarf galaxies that built it. We have used this dataset to show that M31′s stellar halo extends to at least 180 kpc in projection from the center of M31. Furthermore, the density profile of stars shows no indication of a break, even though we are now tracing it to 2/3 of M31′s virial radius (Gilbert et al. 2012).

I am currently using the M31 halo dataset to analyze the metallicity distribution of M31 halo stars as a function of radius. We have spectra of over 1500 M31 stars in 32 spectroscopic fields ranging in distance from 9 to 180 kpc from M31′s center. The data show a clear gradient in metallicity that extends to 100 kpc (Figure 2). This gradient is seen in both the photometric (based on a star’s position in the color-magnitude diagram) and spectroscopic (based on the strength of the Calcium II triplet absorption feature) metallicity estimates.

Our spectra allow us to analyze the velocity distributions of stars in each field and to identify tidal debris features by their cold kinematical signatures. When we remove stars associated with tidal debris, the strength of the observed metallicity gradient increases.

Figure 2: Metallicity Distribution Functions of stars in M31′s halo: (top left) all M31 halo stars; (top right) after removal of tidal debris features; (bottom) cumulative distributions for all halo stars (solid curves) and after removal of tidal debris features (dashed curves). Arrows mark the median [Fe/H] values for each distribution. The inner halo is primarily metal-rich, while the outer halo is significantly more metal-poor. The data show evidence of a metallicity gradient in M31′s stellar halo extending from 9 kpc to ~100 kpc.

This large-scale metallicity gradient, when compared to the results of simulations of stellar halo formation, implies that the bulk of M31′s stellar halo was likely built primarily from one to a few relatively massive dwarf galaxies (>109 solar masses).

However, we also observe significant field-to-field scatter in the mean metallicities and surface brightnesses of fields at large radius. This implies that recently accreted, small dwarf galaxies have contributed substantially to the outermost regions of M31′s stellar halo.

If you are interested in learning more, a paper presenting these results is in progress. It should appear on astro-ph in the next few months! You can also check out other recent SPLASH papers, discussing the extended surface brightness profile of M31 (Gilbert et al. 2012), the properties of the inner regions of M31′s stellar halo (Dorman et al. 2012 and Dorman et al. 2013), and our spectroscopic survey of M31′s dwarf galaxies (Tollerud et al. 2012).

It’s exciting to witness history being made, especially if it’s a long-awaited event. The afternoon of October 6, 1995 was that time. Several hundred astronomers were in Florence, Italy for the ninth in a series of scientific meetings on Cool Stars, Stellar Systems, and the Sun. The Cool Stars series was started by Andrea Dupree in 1980 in Cambridge, Massachusetts, and they continue to be held every two years. Cool Stars 9 was the first to be held outside the U.S., and Roberto Pallavicini from Florence’s Arcetri Observatory led the group that organized and hosted the meeting.

Florence is a delightful place to be under almost any circumstances: warm, sunny, good food, and great sights. But everyone was very focused that afternoon. In the morning we were told that there’d be a special short talk later, and the buzz over lunch was that Michel Mayor of the Geneva Observatory, working with Didier Queloz, had a rock-solid detection of a planet around another Sun-like star to report.

When we returned to the meeting hall after lunch I made sure to get a seat up front. Nobody really knew anything yet, just that a big announcement was coming. We all knew Mayor and Queloz were searching for planets, so what else could it be? But which star, and what kind of planet? All the brighter Sun-like stars were well known to many of us, so it was probably a star we’d heard of and studied ourselves.

We then had to sit through most of the afternoon’s scheduled talks; they were selected to be interesting in the first place, but at this point were being eclipsed by the Main Event. As the time grew near for Mayor’s presentation some people with television cameras started filtering into the back of the room. Mayor showed us his observations and conclusions, and there was little to argue with. There was warm and enthusiastic applause: the prey so long sought had been nabbed.

So why did it take so long? It’s been pointed out that 51 Pegasi, the host star to this planet, was misclassified as being evolved. But that in itself wouldn’t preclude detecting the planet that was found. A big part of the reason is simply that 51 Peg B – the planet – was so very different than anyone ever expected. Or, to be more precise, it was located where we did not think we’d find planets: close in to the host star.

The conventional wisdom was that other planetary systems would be like ours, with analogs to Jupiter at great distances and with long periods that would take years of careful measurements to detect. At the time I imagined the process Mayor and Queloz must have gone through. They’d been observing 51 Peg and stars like it for some time, hoping to see very small changes in the motions of the stars that would indicate a planetary companion. 51 Peg showed larger variations than they could make sense of, so they started observing it more often, maybe every month instead of a few times per year. And still, 51 Peg showed some kind of motion going on but with no clear period. So they started observing it every week, and still no period to the variations. It was only when they measured it every night for a few weeks that the 4.2-day period became blindingly obvious. They had it!

Once the realization dawned that these “hot Jupiters” could exist so close to a star they became fairly easy to find because they produce much larger signals than more distant planets. The race was on and over the next several years there were dozens and dozens of new exoplanets announced by Mayor’s group and a competing group led by Geoff Marcy in California. There was no turning back. History was made.

Scientific meetings like Cool Stars 9 often feature an invited talk at the end by a senior researcher who provides a summary of the high points and his or her views on their significance and implications for the near future. That person that time was Jeff Linsky, from Boulder CO. But Linsky played hooky to visit museums the afternoon that Mayor made his presentation and so he missed it. His conference summary mentions nothing of the one talk that will be remembered longest.

Finally, it is now nearly 20 years since Cool Stars 9. I was conscious of events at the time, but the details are getting fuzzier, and the recollections of others may sound different.

This Month’s Featured Author

Dr. Brian Williams received his B.S. from Florida State University in 2004 and his Ph.D. from North Carolina State University in 2010. He was a NASA Postdoctoral Fellow at NASA Goddard Space Flight Center for three years, after which he worked as a research scientist at NASA GSFC with Universities Space Research Association. He arrived at STScI in February of 2017, and is currently a Support Scientist in the Science Mission Office. His research interests include supernovae and supernova remnants, shock physics and particle acceleration, and dust in the interstellar medium.